The requirements for the signal quality of transmitting devices are becoming more stringent as the need for high data rates and increasing mobility grows. The modern mobile radio standards such as UMTS/WCDMA, GSM/EDGE, 802.11a, 802.11b, 802.11g or Bluetooth Medium Data Rate require special modulation types for data transmission, which modulate both the phase and the amplitude of a carrier signal at the same time. Simultaneous amplitude and phase modulation makes it possible to achieve higher data transmission rates and thus better bandwidth efficiency. The mobile radio standards mentioned above envisage, for example, the use of QPSK (Quadrature Phase Shift Keying, 8-PSK (8-Phase Shift Keying) or QAM (Quadrature, Amplifier, Modulation) as modulation types for the data transmission rate. Depending on the selected application for the individual mobile radio standards, these high-quality modulation types are used not only for data transmission from a base station to a mobile communication appliance but also from the mobile communication appliance to the base station.
The modulation types which are used for modern mobile radio standards are particularly sensitive to possible interference or distortion, which is produced by various components in the transmission path. Interference or distortion such as this in the transmission path leads to changes in the phase and amplitude of a carrier signal. This results in data errors in the transmitted signal.
In order to suppress the interference or distortion, it is necessary for the individual components in the transmission path to have a highly linear transmission characteristic or transmission response. In this example, the expression transmission characteristic linearity means the transmission response of an element within the transmission path, which essentially produces an output signal that is proportional to the input signal. Circuits whose transmission responses have non-linear areas produce an output signal which is not proportional to an input signal. This component characteristic, which is also referred to as non-linearity, can lead to data errors within the transmitted signal.
Typical circuit elements within the transmission path whose characteristic has non-linear areas are, in particular, the individual amplifiers in the transmission path which amplify the signal to be transmitted to the output power level. By way of example, in the example of power amplifiers, a high degree of linearity in their output signal is achieved by operating the power amplifiers considerably below their maximum achievable output power level. This is referred to as operation in the linear area of their characteristic. However, operation of the power amplifier in this way leads to a high quiescent current being drawn, thus increasing the overall power loss. The efficiency, which mainly indicates the ratio of the output power that is produced to the consumed power that is applied to the power amplifier, in consequence falls. Particularly in the example of mobile communication appliances, the greater current that is drawn in the power amplifiers reduces the operating time of the communication appliances, which is governed by the capacity of their rechargeable batteries.
In order to increase the efficiency of the individual power amplifiers, and thus of the overall transmitting device, it is expedient to operate the individual amplifiers and other active circuits in their maximum achievable power range. However, the transmission characteristics of the individual active switching elements have a very highly non-linear response in this range. In consequence, the output signal is considerably distorted, thus possibly inducing data transmission errors.
Modern mobile communication appliances normally attempt to reach a compromise between the current that is drawn and the linearity of the individual active switching elements in the transmission path. This can be achieved by suitable circuitry. By way of example, it is possible to reduce the current that is drawn by choice of suitable biasing, the adjustment of the operating points and by a suitable load impedance of the output of the components with a non-linear characteristic. The documents by G. L. Madonna et al.: “Investigations on Linearity characteristics for large-emitter area GaAs HBT power stages”, GAAS 2001 conference, London 2001 and Iwai et al.: “High efficiency and high linearity InGaP/GaAs HBT power amplifiers: Matching techniques of source and load impedance to improve phase distortion and linearity” IEEE transaction on electronics devices, volume 45, No. 6, June 1998 disclose various examples for a method such as this. In order to further improve the transmission response of the overall transmission path and in order to reduce possible data errors, it is normal in modern transmitting devices to additionally predistort the input signal.
In the example of predistortion, an improvement in the signal quality is achieved by supplying a distorted signal to the amplifier, or to the component with the non-linear characteristic. The distortion is in this example chosen such that the distortion caused by the transmission response is accurately compensated for. It is then once again possible to tap off a signal which is approximately proportional to the input signal at the output of the amplifier or of the component with a non-linear characteristic.
The documents Yamauchi et al.: “A Novel Series Diode Linearizer for Mobile Radio Power Amplifiers”, IEEE MTT-S 1996, pages 831 to 833 an E. Westesson et al.: “A Complex Polynomial Predistorted Chip in CMOS for Baseband or IF Linearization for RF Power Amplifiers”, IEEE Internations Symposium of Circuits and Systems 1999, describe examples of predistortion within an analog signal processing chain in the transmitting device. Circuits for distortion of analog signals can be produced at particularly low cost by means of simple additional elements. However, external operating conditions, only some of which can be influenced, such as the temperature, drive level of the components and operating points of the individual circuits, may be varied only within narrow limits. Otherwise, additional readjustment of the predistortion circuit is required. Additional control circuits for predistortion of analog signals require additional space on a semiconductor body, and increase the current that is drawn. Furthermore, they lead to only moderate improvements in terms of the linearity of an output signal.
In contrast to this, predistortion of digital signals offers very good adaptability to changing external operating conditions. In this example, predistortion is carried out by a variation of the so-called digital baseband signals. In this example, even before conversion to an analog baseband signal or before modulation of a carrier signal, the baseband signal is changed in such a way as to compensate for the distortion caused by the circuits with a non-linear characteristic. In the example of so-called adaptive digital predistortion, a portion of the analog output signal is extracted downstream from the elements with a non-linear characteristic, is demodulated and is converted back to a digital baseband signal.
The distortion caused by the components with a non-linear characteristic within the transmission path can be determined from the comparison of the converted baseband signal with the original undistorted digital baseband signal. The documents U.S. Pat. Nos. 6,477,477 and 4,291,277 disclose examples of transmitting devices with adaptive digital predistortion. In the example of mobile communication appliances, which in particular are intended to cost little and are designed to be small and current-saving, this procedure is unattractive in some circumstances, since the computation complexity that is required in the digital area is relatively high.